Air separation by cryogenic means involves the use of cold generation and of a cold source.
It is known to expand, with external work, gases under pressure which are introduced into an expansion machine at temperatures appreciably above their dew point.
FR-A-2,335,809 describes a single-turbine apparatus which provides the process with all the necessary refrigeration. The gas expanded in the turbine may be medium-pressure nitrogen or air. Overpressurized air is liquefied by heat exchange with liquid oxygen under pressure, which vaporizes.
U.S. Pat. No. 5,564,290 describes a process in which pressurized air, condensed by the vaporization of pumped liquid oxygen, then vaporizes in a turbine so as to produce a two-phase flow.
It is also known to keep an apparatus cold, at least partially, by feeding cryogenic liquids into distillation columns.
Known hydraulic turbines produce a fluid which is generally in liquid form.